Trocars are sharp-pointed instruments used to puncture a body cavity. A body cavity is often punctured so that fluids may be drained using a cannula inserted into the opening. Trocars are also used during endoscopic procedures. A conventional endoscopic procedure follows three steps. The first step is the insertion of a Veress cannula into an abdominal cavity through a small incision in the abdominal wall. The second step is the inflation of the cavity with insufflating gas passed through the cannula. After inflation, the Veress cannula is removed. The third step is the thrusting of standard trocar housed within the bore of a trocar tube into the inflated abdomen. Standard trocars are shaped like a large metal peg with a sharpened point. The trocar is then removed, and the endoscopic instrument is inserted into the abdominal cavity through the trocar tube.
U.S. Pat. Nos. 4,601,710 (the '710 patent) and 4,654,030 (the '030 patent), the disclosures of which are hereby incorporated by reference, describe embodiments of a trocar assembly having a spring-biased tubular protective shield. One of the embodiments in the '710 patent has a shield locking mechanism that comprises a slide valve-actuated locking tooth that engages a slot in the wall of the shield. The '030 patent discloses an embodiment wherein the flap valve functions as a shield locking means wherein a tip seats against a recessed shoulder on the shield. The valve is manually controlled to release the shield.
Trocar assemblies having spring-biased tubular protective shields include a tubular protective shield within a trocar tube. In addition, once the piercing tip of the obturator penetrates a wall of a body cavity, the protective shield must further widen the opening created by the piercing tip before the protective shield can move outwardly of the trocar tube to shield the piercing tip which remains in the body cavity. U.S. Pat. No. 4,535,773 (the '773 patent) discloses a reusable trocar wherein the obturator can be retracted by means of a spring and an electromechanical latching mechanism. In particular, the piercing tip of the obturator includes pressure sensing means which transmits signals to activate a solenoid which in turn releases the obturator shaft so that it can be retracted. The retractable obturator disclosed in the '773 patent requires a complex arrangement wherein sliding contacts and circuitry are used to energize a solenoid which removes a detent which is biased into engagement with a sliding part of the obturator. Such an arrangement is costly and could be unreliable due to bad connections in the electrical circuitry.
It would be desirable to provide a disposable trocar assembly wherein the piercing tip can be retracted automatically after piercing the wall of the body cavity by a purely mechanical arrangement. Such a retractable tip trocar assembly would provide advantages over the tubular protective shield arrangement of the '710 and '030 patents since the trocar tube could be used to shield the obturator, thereby eliminating the need for the protective tubular shield.